1. Field of the Invention
The disclosure relates a method and a system for reducing the vertical motions on floating platforms for drilling and production. More particularly, the present disclosure relates to floating platforms used in the exploration and production of offshore oil and gas, and more particularly to a semisubmersible floating platform having a keel plate to function as a heave plate.
2. Description of the Related Art
With the significantly increasing demand on the oil and gas supply, offshore exploration and production from reservoirs has become vital to such supply. These reservoirs usually require large drilling rigs and variable payloads which result in very large topsides in both size and weight. Large and expensive supporting offshore platforms are needed. However, the expense of such platforms can be decreased by building such a floating structure near or on shore and towing the structure to the intended offshore site.
Among the main types of offshore platforms designed for deep water, including the popular Spar, a type of platform is known as a semi-submersible platform. The structure is built near shore or onshore, floated to the offshore site, and partially submerged using ballast tanks to provide stability to the structure. Semi-submersibles are typically configured with large buoyant pontoon structures below the water surface and slender columns passing through the water surface supporting a topsides deck at a significant height above the water surface. Semi-submersible platforms make large and cost effective platforms for drilling and production of offshore oil and gas. However, because the structure has a relatively large floating surface, one challenge is restricting movement caused by wave and wind action to provide a desired stability for operations.
Heave plates have been used to stabilize movement of the semi-submersible platforms. The heave plate can be a solid plate or a constructed assembly of a plurality of plates that form a box to form a relatively large horizontal surface area, but is relatively thin vertically. The heave plate is mounted to the semi-submersible platform below the water surface and below at least a portion of the wave-influenced water zones. The heave plate increases the hydrodynamic mass of the offshore platform, where hydrodynamic mass is a measure of the amount of a fluid moving with a body that accelerates in the fluid and depends on the shape of the body and the direction of its motion. The heave plate at the lower depths provides additional resistance to vertical and tilting motion that would otherwise occur near or at the water surface. Thus, designers are motivated to mount the heave plate at deeper levels. However, the depth is initially limited, because the platform is built near or on shore at shallow depths. Thus, some systems have a lowering capability to the heave plate. The heave plate can be lowered to a more desirable depth after the platform is in position at the intended offshore site. Examples of such systems are illustrated, for example, in U.S. Pat. No. 6,652,192, U.S. Pat. No. 7,219,615 (as a continuation of U.S. Pat. No. 7,156,040), and U.S. Pat. No. 6,718,901, and are incorporated by reference herein. Each of these systems discloses lowering the heave plate to a depth below the platform after being located to the intended offshore site.
U.S. Pat. No. 6,652,192 discloses a heave suppressed, floating offshore drilling and production platform having vertical columns, lateral trusses connecting adjacent columns, a deep-submerged horizontal plate supported from the bottom of the columns by vertical truss legs, and a topside deck supported by the columns. The lateral trusses connect adjacent columns near their lower end to enhance the structural integrity of the platform. During the launch of the platform and towing in relatively shallow water, the truss legs are stowed in shafts within each column, and the plate is carried just below the lower ends of the columns. After the platform has been floated to the deep water drilling and production site, the truss legs are lowered from the column shafts to lower the plate to a deep draft for reducing the effect of wave forces and to provide heave and vertical motion resistance to the platform. Water in the column shafts is then removed for buoyantly lifting the platform so that the deck is at the desired elevation above the water surface.
U.S. Pat. No. 7,219,615 discloses a semi-submersible vessel having a pair of vertically spaced pontoons with varied buoyancy. The lower pontoon is retained in a close vertical proximity to the upper pontoon when the vessel is in transit. The lower pontoon is ballasted at the deployment site, dropping the pontoon to a depth of about 32 meters below the first pontoon baseline. As a result, stability and motion characteristics of the vessel are significantly improved.
While each of these systems offer solutions for a stabilized platform having a lowered heave plate, in practice the supporting structure for the heave plate to the platform may suffer from rigidity challenges. For example, U.S. Pat. No. 7,219,615 discloses extendable legs. Due to the extendable nature of the legs, no diagonal bracing between legs is shown that would be able to resist twisting and bending of the extended support structure to the heave plate, because diagonal bracing between the legs would apparently interfere with extending and retracting the legs through the guides. U.S. Pat. No. 6,652,192 illustrates extendable trusses within columns having diagonal flexible cable bracing installed between trusses after extension of the legs. Due to an interference between the truss diagonal members and the column, it is hard to design a receptacle which can enclose the truss legs and rigid diagonal bracing for effective support and load transfer. The patent does not disclose rigid bracing between trusses for the same reason, namely, the rigid bracing between the trusses would appear to interfere with extending and retracting the trusses. Another example includes U.S. Pat. No. 6,718,901 that discloses extendable legs so that deploying an offshore oil and gas production platform comprises placing a buoyant equipment deck on a buoyant pontoon so that elongated legs on the pontoon, each comprising a buoyant float, extend movably through respective openings in the deck. Chains extending from winches on the deck are reeved through fairleads on the pontoon and connected back to the deck. The chains are tightened to secure the deck to the pontoon for conjoint movement to an offshore location. The chains are loosened and the pontoon and leg floats ballasted so that the pontoon and leg floats sink below the floating deck. A further example of the extending draft concept is seen in US Publ. No. 20020041795.
Further, a deep draft semisubmersible usually needs to have larger than a 60 m draft to have the favorable motion to support the connections to the sea floor in harsh sea states. With this deep draft semisubmersible, the topside integration at the quayside and the transition from the fabrication yard to the installation site become problematic, because the column is too high to stabilize the platform during the transition mode. Many designs solve this difficulty by extending the draft that requires the significant risk of offshore installation operation.
There remains a need for a different system and method for a floating offshore platform having an improved stabilization of the offshore platform.